Solenoid-type fuel injectors typically are assembled by welding together non-stabilized ferritic stainless steel components, such as a metal tubular injection body welded to a metal fuel tube. Such welding may be carried out by various techniques, including microplasma welding, but typically employs a laser with a continuous laser beam to melt and fuse the components at the bonding surfaces, forming multiple laser weld sites. A slipover injection molded plastic sleeve typically is employed to cover the weld sites; however, this can enhance corrosion at the covered weld sites by fostering an oxygen depletion condition.
The process of laser welding two metal components together heats a zone at and below the touching surfaces to create a pool of molten metal within both members that, upon solidification, forms the weld nugget that joins the two metallic components. Because of the high temperatures inherent in any laser welding process, non-stabilized ferritic stainless steel components and the like used in the production of solenoid-type fuel injectors can incur a deleterious metallurgical effect known as sensitization.
Metallurgical sensitization is characterized by intergranular corrosive precipitates, for example, chromium carbides, that render the weld site and surrounding metallic area susceptible to intergranular attack and corrosion. Non-stabilized ferritic stainless steel grades typically used in conventional solenoid-type fuel injectors incur metal sensitization upon being welded, and are thus rendered susceptible to intergranular corrosive attack. The sensitized weld sites are vulnerable to corrosion that can lead to failure of the fuel injector. Because of this susceptibility, weld sites in conventional prior art solenoid-type fuel injectors typically are hermetically sealed, an expensive and time consuming process, to prevent them from contacting detrimental corrosive environments such as those typically experienced under the hood of automotive vehicles.
Further, non-stabilized ferritic stainless steels used in prior art fuel injectors typically are not formulated for easy, or “free,” machining. Components therefore are formed typically by stamping, or by machining only with great difficulty and expense.
Therefore, there is a strong need for an improved solenoid-type fuel injector assembly comprising components that are easily weldable, can be easily formed by machining, are stabilized against metallurgical sensitization and corrosive attack, exhibit soft magnetic properties capable of carrying a magnetic flux, and are able to withstand harsh corrosive environments.
It is a principal object of this invention to provide a solenoid-type fuel injector assembly having components which are laser weldable and are also capable of withstanding harsh corrosive environments.
It is a further object of this invention to provide a solenoid-type fuel injector assembly comprising stainless steel components that are readily formed by machining.
It is a still further object of this invention to provide a solenoid-type fuel injector assembly having reduced susceptibility to metallurgical sensitization and intergranular corrosive attacks at laser weld sites joining the components.
It is a still further additional object of this invention to provide a solenoid-type fuel injector assembly having ferritic stainless steel polepieces that exhibit soft magnetic properties and are thus capable of carrying a temporary magnetic flux.
The inventor has developed an improved solenoid-type fuel injector assembly capable of solving the aforementioned problems by the incorporation of components formed from or welded to stabilized ferritic stainless steel, preferably a solenoid-quality and preferably a “free machining” (FM) grade of stabilized ferritic stainless steel, which may be laser-welded.